1. Field of the Invention
The present invention relates to a semiconductor apparatus and a fabrication method thereof, more particularly, a semiconductor apparatus, for example, a solid-state image pickup device such as a charge coupled device (CCD) including interlayer lenses, a liquid crystal display device, and the like, and a fabrication method thereof.
2. Description of the Related Art
A metal oxide semiconductor (MOS) type solid-state image pickup device such as a CCD is used in an electronic information tool for various applications such as a digital camera, a video camera, a cell phone with a built-in camera, a scanner, a digital copying machine, a facsimile, and the like.
As electronic information tools, including a semiconductor apparatus such as a solid-state image pickup device, becomes popular in the market, there is a growing demand for a solid-state image pickup device with small size and low cost, as well as enhanced functions and high performances, for example, a large number of pixels, an improved light-receiving sensitivity or the like.
In order to meet such a demand, the size of the solid-state image pickup device is reduced and the pixels are formed with a high density. As this continues, the size of each of the pixels incorporated into the solid-state image pickup device will be reduced. As a result, a light-receiving sensitivity of the solid-state image pickup device may be deteriorated and the high light-receiving sensitivity, i.e., one of the basic performances of the solid-state image pickup device, cannot be achieved. It may be difficult to shoot a sharp image at a certain illuminance.
As a resolution for such a problem, for example, Japanese Laid-Open Publication No. 4-12568 discloses a solid-state image pickup device having a microlens formed of an organic high polymer material on a color filter in order to improve the light-receiving sensitivity. Japanese Laid-Open Publication Nos. 2000-164837 and 11-40787 disclose a solid-state image pickup device having a microlens formed on a color filter and also an interlayer lens formed in a laminate structure provided between the color filter and a light-receiving portion to further improve the light-receiving sensitivity.
Hereinafter, an example of a conventional CCD solid-state image pickup device having interlayer lenses will be described.
FIG. 4 is a schematic cross-sectional view of a pixel 250 of a conventional CCD solid-state image pickup device 200.
Note that, although the CCD solid-state image pickup device 200 includes a plurality of the pixels 250, only one pixel is shown in the figures for the sake of clarity.
In the CCD solid-state image pickup device 200 shown in FIG. 4, charge transferring portions, i.e., a plurality of CCD transfer channels 104 are provided in an upper portion of a semiconductor substrate 101 with predetermined spaces therebetween. In the spaces between the CCD transfer channels 104 next to each other, a plurality of light receiving portions 102 having a photoelectric conversion function are buried with appropriate spaces from the CCD transfer channels 104. Between the light receiving portions 102 corresponding to pixels 250 and the CCD transfer channels 104 on one side, readout gate portions 103 are respectively provided. Between the light receiving portions 102 and the CCD transfer channels 104 on the other side, channel stopper portions 105 are respectively buried.
In the upper portion of the semiconductor substrate 101, one light receiving portion 102 and the CCD transfer channel 104 with the readout gate portion 103 interposed therebetween are separated from another light receiving portion 102 of an adjacent pixel 250 and the CCD transfer channel 104 by the channel stopper portion 105. An insulating film 106 is provided across the entire surface of the semiconductor substrate 101, covering the light receiving portions 102, the CCD transfer channels 104, the readout gate portions 103, and the channel stopper portions 105.
A transfer electrode 107 is provided on the insulating film 106 for each of the CCD transfer channels 104. The transfer electrodes 107 are covered with interlayer insulating films 108. On the insulating film 106, a light shield film 109 is provided so as to cover the interlayer insulating films 108. The light shield film 109 blocks the incidence of light onto the transfer electrodes 107.
The light shield film 109 includes: portions 109a which are respectively provided above portions of the CCD transfer channels 104 closer to the readout gate portions 103; portions 109b which are respectively provided above the readout gate portions 103 and above portions of the light receiving portions 102 closer to the readout gate portions 103 and which contact the insulating film 106; portions 109c which are respectively provided above the channel stopper portions 105 and above portions of the light receiving portions 102 closer to the channel stopper portion 105 and which contacts the insulating film 106; and portions 109d which are respectively provided above portions of the CCD transfer channels 104 closer to the channel stopper portions 105.
In regions between the transfer electrodes 107 next to each other, the portions 109b and the portions 109c directly contact the flat insulating film 106.
In the light shield film 109, the portions 109a and the portions 109b have a difference in levels of height and form step portions, and the portions 109c and the portions 109d have a difference in levels of height and form step portions.
The light shield film 109 has openings 109x. Each of the openings 109x corresponds to portions of the light receiving portions 102. The light receiving portions 102 have opening regions 102x. In the openings 109x, the opening regions 102x of the light receiving portions 102 are exposed via the insulating film 106. Light impinges on the opening regions 102x of the light receiving portions 102 through the openings 109x provided in the light shield film 109.
A first flattening film 110 formed of a boro-phospho silicate glass (BPSG) film is provided on the light shield film 109 by, for example, a normal-pressure CVD process. The first flattening film 110 has a surface shape which corresponds to a shape of the light shield film 109. The portions of the surface above the light receiving portions 102 are concaved.
A lens forming layer 111 formed of a high-refractive-index material such as a silicon nitride film is provided on the first flattening film 110. Interlayer lenses 111a are respectively provided in the concave portions of the first flattening film 110. The interlayer lenses 111a are located above the light receiving portions 102. Each of the interlayer lenses 111a has a lens surface of a convex shape protruding downward on the lower side and also has a lens surface of a convex shape protruding upward on the upper side. The lens forming layer 111 is flattened except for the portions where the interlayer lenses 111a are provided.
A second flattening film 112 formed of a low-refractive-index material is provided on the lens forming layer 111. A surface of the second flattening film 112 is flattened. A color filter 113 and a protection film 114 of a uniform thickness are sequentially provided on the second flattening film 112.
A plurality of microlenses 115 for converging incidence light to the opening regions 102x of the light receiving portions 102 are provided on the protection film 114 so as to respectively correspond to the light receiving portions 102. Each of the microlenses 115 is a convex lens having the upper surface protruding upward such that the center portion is thicker than the peripheral portion. Each of the microlenses 115 is located so as to cover a corresponding light receiving portion 102, and portions of the CCD transfer channels 104 provided on both sides of the light receiving portion 102.
Next, a method for fabricating a conventional CCD solid-state image pickup device 200 shown in FIG. 4 will be described.
FIGS. 5A through 5E are cross-sectional views respectively showing the steps in the method for fabricating a conventional CCD solid-state image pickup device 200.
With reference to FIG. 5A, required impurities are added to the semiconductor substrate 101 by an ion implantation or the like. Thus, a plurality of the CCD transfer channels 104 are formed in an upper portion of the semiconductor substrate 101 with predetermined spaces therebetween. Also, the light receiving portions 102 are respectively provided between the CCD transfer channels 104 with appropriate spaces therefrom in the upper portion of the semiconductor substrate 101. Further, the channel stopper portions 105 are respectively formed between the light receiving portions 102 and the CCD transfer channels 104 on one side. Regions between the light receiving portions 102 and the CCD transfer channels 104 on the other side function as the readout gate portions 103. One light receiving portion 102 and the CCD transfer channel 104 with the readout gate portion 103 interposed therebetween are formed in the upper portion of the semiconductor substrate 101 being separated from another light receiving portion 102 of an adjacent pixel 250 and the CCD transfer channel 104 by the channel stopper portion 105.
Next, the insulation film 106 formed of, for example, SiO2 or the like, is formed on a surface of the semiconductor substrate 101 by a thermal oxidation process or a CVD process.
Then, a polysilicon film, for example, is formed on the insulating film 106 by a CVD process. The formed polysilicon film is patterned by photolithography or etching to form transfer electrodes 107 formed of the polysilicon film such that they respectively correspond to the CCD transfer channels 104. Then, the interlayer insulating films 108 formed of, for example, SiO2 or the like, are formed by a CVD process and so on so as to respectively cover the transfer electrodes 107.
Next, the light shield film 109 formed of a high-melting-point metal such as titanium (Ti), tungsten (W), or the like, is formed by a sputtering process so as to cover the transfer electrodes 107 covered by the interlayer insulating films 108 and the insulating film 106 between the transfer electrodes 107 next to each other. As described above, the light shield film 109 includes the portions 109a, the portions 109b, portions 109c and the portions 109d. The step portions are formed between the portions 109a and the portions 109b and the step portions are formed between the portions 109c and the portions 109d. 
Then, the light shield film 109 formed of a high-melting-point metal is patterned by photolithography and etching. Thus, the openings 109x are formed to respectively correspond to the regions of the light receiving portions 102.
Next, with reference to FIG. 5B, the BPSG film having phosphorous (P) and boron (B) of a predetermined concentration is deposited by, for example, a normal-pressure CVD process on the light shield film 109 having the openings 109x. Then, a reflow process is performed under a high temperature of 900° C. or higher. Thus, the first flattening film 110 formed of the BPSG film is formed. The portions of the surface of the first flattening film 110, i.e., the BPSG film, are concaved where the openings 109x are provided above the light receiving portions 102 due to the step portions in the light shield film 109.
Next, with reference to FIG. 5C, the lens forming layer 111 formed of a high-refractive-index material such as a silicon nitride film is formed on the first flattening film 110 by, for example, a plasma CVD process. A surface of the lens forming layer 111 is flattened.
Next, a resist 117 is applied on the lens forming layer 111 to have a predetermined thickness. The resist 117 is patterned so as to be left at a position corresponding to the light receiving portion 102 and the portions around the light receiving portion 102. Then, a reflow process is performed, for example, at a temperature around 160° C. Accordingly, as shown in FIG. 5D, the resists 117 have convex-lens shapes protruding upward such that the central portion is thicker than the peripheral portion.
Next, using the resists 117 of convex-lens shapes as a mask, the lens forming layer 111 is etched by dry-etching. By setting an appropriate selective etching ratio for the resists 117 and the lens forming layer 111, the surface of the lens forming layer 111 is etched to have a convex-lens shape protruding upward similarly to the resists 117 of convex-lens shapes as shown in FIG. 5E. Thus, the interlayer lenses 111a are formed in the positions opposing the light receiving portions 102. Each of the interlayer lenses 111a has a lens surface of a convex shape protruding downward on the lower side and also has a lens surface of a convex shape protruding upward on the upper side. The lens forming layer 111 is flattened and has a predetermined thickness except for the portions where the interlayer lenses 111a are formed.
On the lens forming layer 111 provided with the interlayer lenses 111a, a second flattening film 112 formed of a low-refractive-index material is formed in order to improve the converging rate of the interlayer lenses 111a. The second flattening film 112 is formed so as to cover the lens forming layer 111 and a surface thereof is flattened (see FIG. 4).
Then, a color filter 113 and a protection film 114 of a predetermined thickness are sequentially provided on the second flattening film 112. A plurality of microlenses 115 for converging incidence light to the light receiving portions 102 are provided on the protection film 114. Each of the microlenses 115 is formed at a position opposing one light receiving portion 102, and portions of the CCD transfer channels 104 provided on both sides of the light receiving portion 102 (see FIG. 4). The microlenses 115 are formed to have a shape of a convex lens having the upper surface protruding upward such that the center portion is thicker than the peripheral portion.
In this way, the CCD solid-state image pickup device 200 shown in FIG. 4 is obtained.
In such a method for fabricating the CCD solid-state image pickup device 200, as described above with respect to FIG. 5B, the BPSG film including boron (B) and phosphorous (P) is processed with there flow process under high temperature. When such a process is performed, the surface of the first flattening film 110 is concaved by using the step portions in the light shield film 109. The lens surfaces on the lower side of the interlayer lenses 111a are formed corresponding to the concaved surface of the first flattening film 110. Thus, the central position of the lens surface of the lower side of an interlayer lens 111a may be undesirably deviated from the central axis of the corresponding opening region 102x, i.e., the central axis of the corresponding opening 109x. 
The details will be explained below.
In the light shield film 109, the portions 109b of the light shield film 109 and the portions 109c of the light shield film 109 both overlay the corresponding light receiving portions 102. However, the lengths of the portions 109b are different from those of the portions 109c. For example, in the case of the CCD solid-state image pickup device 200, the portions 109c of the light shield film 109 are usually longer than the portions 109b of the light shield film 109 in order to securely block the channel stopper portions 105 from the light. Accordingly, the portions 109b and the portions 109c are formed in asymmetrical positions with respect to the central portion of the light receiving portions 102.
The central line A-A′ runs through the center C of each of the concaved portions on the surface of the first flattening film 110 after the reflow process which is decided based on the positions of the step portions between the portions 109a and portions 109b of the light shield film 109 and the step portions between the portions 109c and portions 109d of the light shield film 109. The central axis B-B′ runs through the central portion of the corresponding opening 109x above the light receiving portion 102. The central line A-A′ is not aligned with the central axis B-B′. The center C′ of the lens surface on the upper side of each of the interlayer lenses 111a is formed so as to align with the central axis B-B′ running through the central axis of each of the openings 109x, i.e., that of each of the opening regions 102x. 
Therefore, the optical axis of each of the interlayer lenses 111a, i.e., the line connecting the center C of each of the lens surface on the lower side of each of the interlayer lenses 111a and the center C′ of the lens surface on the upper side of each of the interlayer lenses 111a is not aligned with the central axis of the corresponding opening region 102x nor is parallel thereto. As a result, the light converged by the interlayer lenses 111a does not pass through the openings 109x efficiently. The amount of light received by the light receiving portions 102 may decrease. The desirable light receiving sensitivity may not be obtained for the CCD solid-state image pickup device 200.
In the method described above, the BPSG film is subjected to the reflow process under high temperature, and the lens surfaces of the convex-shape protruding downward on the lower side of the interlayer lenses 111a are defined based on the concaved portions on the surface of the first flattening film 110. Such a method has a further problem that lens surfaces of the convex-shape on the lower side of the interlayer lenses 111a may not be stably and uniformly formed. More specifically, the lens surfaces on the lower side of the interlayer lenses 111a are defined by the concaved surface of the first flattening film 110, including the BPSG film which is subjected to there flow process under the high temperature. Thus, the lens surfaces on the lower side of the interlayer lenses 111a depend on the concentrations of boron (B) and phosphorous (P) in the BPSG film, the temperature for the reflow process, and the feature of the underlying surface of the light shield film 109 having the step portions. As a result, the converging rate of the interlayer lenses 111a may decrease, causing deterioration in the image quality of the CCD solid-state image pickup device 200.